People get alopecia through several different pathways, and the cause depends on which type of alopecia is involved. Some forms result from the immune system attacking hair follicles, others from hormones shrinking them over time, and still others from physical damage, stress, or nutritional gaps. Understanding the specific cause matters because it determines whether the hair loss is temporary or permanent, and what can be done about it.
Autoimmune Attack on Hair Follicles
Alopecia areata, the form most people mean when they say “alopecia,” is an autoimmune condition. About 2% of people worldwide will experience it at some point, and roughly 700,000 Americans have some form of it at any given time. It happens when immune cells mistakenly identify hair follicles as threats and attack them.
Normally, hair follicles have a kind of immune protection that shields them from the body’s defense system. In alopecia areata, that protection breaks down. Once it does, immune cells (primarily a type called CD8+ T cells) swarm the follicle in a pattern pathologists describe as a “swarm of bees.” These cells, along with other immune players like natural killer cells and specialized white blood cells, create intense inflammation around the follicle that disrupts hair growth and causes the hair to fall out, often in smooth, round patches.
Nobody fully understands why this immune malfunction starts. Genetics play a role, as the condition runs in families. Stress, viral infections, and other environmental triggers can set it off in people who are already predisposed. In many cases, the follicles aren’t destroyed, just suppressed, which is why hair can regrow on its own or with treatment.
Hormones That Shrink Hair Follicles
The most common type of hair loss overall is androgenetic alopecia, better known as male or female pattern baldness. This affects roughly half of men and a significant proportion of women by middle age. The mechanism is hormonal rather than immune-driven.
A hormone called DHT (a potent form of testosterone) binds to receptors on hair follicles, particularly those on the top of the scalp. Over time, this causes the follicles to shrink in a process called miniaturization. Each growth cycle produces a thinner, shorter, lighter hair until the follicle eventually stops producing visible hair altogether. Research in animal models confirms that DHT induces early hair regression, miniaturization, and density loss, and that blocking the hormone’s receptor can partially reverse these effects.
How much DHT your follicles are exposed to, and how sensitive they are to it, is largely genetic. This is why pattern baldness runs strongly in families. Both sides of the family contribute, not just the mother’s side as the old myth suggests.
Stress and Illness That Disrupt the Growth Cycle
Your hair grows in cycles. At any given time, about 85 to 90% of your hair is in the active growth phase, while the rest is in a resting or shedding phase. A major physical or emotional shock can push a large percentage of hairs into the shedding phase all at once. This condition, called telogen effluvium, typically causes diffuse thinning across the entire scalp rather than bald patches.
Common triggers include high fever, severe infections, major surgery, psychological stress, and sudden hormonal changes. Thyroid disorders (both overactive and underactive) are also well-established causes. The hair loss usually doesn’t start at the time of the stressor. Instead, there’s a delay of two to three months, which often makes it hard to connect the shedding to its cause.
The good news is that telogen effluvium is almost always temporary. Once the triggering event resolves, most people see their hair return to normal over six to twelve months. However, if the underlying trigger persists (chronic stress, an untreated thyroid condition, ongoing nutritional deficiency), the shedding can continue.
Postpartum Hormonal Shifts
Pregnancy-related hair loss is a specific and very common form of telogen effluvium. During pregnancy, elevated estrogen levels extend the growth phase of hair, which is why many women notice thicker, fuller hair while pregnant. After delivery, estrogen drops sharply back to normal levels. This causes all that “extra” hair that was held in the growth phase to enter the shedding phase simultaneously.
The result can be alarming. Clumps of hair in the shower or on the pillow are common starting around two to four months after delivery. Despite how dramatic it looks, this shedding is temporary and not a sign of a deeper problem. Most women see full recovery within six to twelve months postpartum.
Nutritional Deficiencies
Hair follicles are metabolically active and need a steady supply of nutrients to function. When levels of key nutrients drop too low, the body essentially deprioritizes hair growth in favor of more critical functions.
Iron is the best-studied link. Research on women of childbearing age uses a serum ferritin level of 10 to 15 ng/mL as the threshold below which hair shedding becomes likely, and this cutoff is highly specific (about 98%) for identifying iron-related hair loss. Vitamin D deficiency has also been associated with diffuse hair loss, though a precise threshold hasn’t been established. Protein deficiency, common with restrictive or fad diets, is another recognized trigger. The pattern of loss is usually diffuse thinning rather than patchy bald spots.
Physical Damage From Hairstyles
Traction alopecia develops when hairstyles repeatedly pull on hair follicles over weeks, months, or years. The constant tension damages the follicle, and the hair loss typically appears along the hairline or wherever the pulling is strongest.
According to the American Academy of Dermatology, hairstyles linked to traction alopecia include tight cornrows, locs, tightly braided hair, high buns and ponytails pulled snugly, hair extensions or weaves (especially on chemically relaxed hair), and rollers worn to bed regularly. Hats, headscarves, and other head coverings can also contribute, particularly when hair is pulled back tightly underneath.
A useful rule of thumb: if a hairstyle feels painful, it’s too tight. Caught early, traction alopecia is reversible. If the pulling continues for years, the follicle damage becomes permanent. The hair loss tends to be gradual, so people often don’t notice it until significant thinning has already occurred.
Scarring Alopecia
Scarring (cicatricial) alopecia is a group of conditions where inflammation destroys the hair follicle and replaces it with scar tissue. This form of hair loss is permanent because the stem cells and oil glands in the middle of the follicle, both essential for generating new hair, are destroyed.
There are two broad categories. Primary scarring alopecia results from inflammatory or autoimmune conditions that directly target the follicle. Secondary scarring alopecia results from external damage to the skin, such as burns, radiation, severe infections, or tumors. In both cases, the affected areas of scalp typically appear smooth and shiny, with no visible follicle openings.
Medications That Cause Hair Loss
A number of common medications can trigger hair shedding as a side effect. Chemotherapy drugs are the most well-known example, but several everyday medications can also contribute. These include certain blood pressure medications (beta-blockers and calcium channel blockers), acne treatments (particularly retinoids, where higher doses carry greater risk), antidepressants, and some anti-inflammatory drugs. Stopping birth control pills can also trigger temporary shedding due to the sudden hormonal shift.
Medication-related hair loss is usually reversible once the drug is discontinued or the dose is adjusted, though it can take several months for regrowth to become noticeable. If you suspect a medication is causing hair loss, the next step is discussing alternatives with whoever prescribed it rather than stopping on your own.